how can I double an image for UV mapping?
I don't think you can duplicate the image within the blender UV editor. However, you can as you said create another image using GIMP. In the UV editor go to UVs > Export UV layout to have an image of your UVs. Then you import it in GIMP and you can edit it. That way, you can be sure it will fit !
You can also make your 2 UV islands overlap in the UV map of your moth. It is pretty quick and you just have to reimport your model in Unity, but I don't think it is something to do when you are making a game (something to do with lightmaps or something like this....).
I am not sure I am being clear enough ...

Hope it will help !